Add support rotate and scale to MP4 stream

pkg/mp4/helpers.go

@@ -1,8 +1,11 @@
 package mp4
 
 import (
-	"github.com/AlexxIT/go2rtc/pkg/core"
+	"bytes"
+	"encoding/binary"
 	"strings"
+
+	"github.com/AlexxIT/go2rtc/pkg/core"
 )
 
 // ParseQuery - like usual parse, but with mp4 param handler
@@ -99,6 +102,67 @@ func ParseCodecs(codecs string, parseAudio bool) (medias []*core.Media) {
 	return
 }
 
+// PatchVideoRotate - update video track transformation matrix.
+// Rotation supported by many players and browsers (except Safari).
+// Scale has low support and better not to use it.
+// Supported only 0, 90, 180, 270 degrees.
+func PatchVideoRotate(init []byte, degrees int) bool {
+	// search video atom
+	i := bytes.Index(init, []byte("vide"))
+	if i < 0 {
+		return false
+	}
+
+	// seek to video matrix position
+	i -= 4 + 3 + 1 + 8 + 32 + 8 + 4 + 4 + 4*9
+
+	// Rotation matrix:
+	// [   cos   sin     0]
+	// [  -sin   cos     0]
+	// [     0     0 16384]
+	var cos, sin uint16
+
+	switch degrees {
+	case 0:
+		cos = 1
+		sin = 0
+	case 90:
+		cos = 0
+		sin = 1
+	case 180:
+		cos = 0xFFFF // -1
+		sin = 0
+	case 270:
+		cos = 0
+		sin = 0xFFFF // -1
+	default:
+		return false
+	}
+
+	binary.BigEndian.PutUint16(init[i:], cos)
+	binary.BigEndian.PutUint16(init[i+4:], sin)
+	binary.BigEndian.PutUint16(init[i+12:], -sin)
+	binary.BigEndian.PutUint16(init[i+16:], cos)
+
+	return true
+}
+
+// PatchVideoScale - update "Pixel Aspect Ratio" atom.
+// Supported by many players and browsers (except Firefox).
+// Supported only positive integers.
+func PatchVideoScale(init []byte, scaleX, scaleY int) bool {
+	// search video atom
+	i := bytes.Index(init, []byte("pasp"))
+	if i < 0 {
+		return false
+	}
+
+	binary.BigEndian.PutUint32(init[i+4:], uint32(scaleX))
+	binary.BigEndian.PutUint32(init[i+8:], uint32(scaleY))
+
+	return true
+}
+
 const (
 	stateNone byte = iota
 	stateInit